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  • Computing stereo channels from masking data.

    24 October 2018

    The detection of stereoscopic depth in random-dot patterns that have been spatially band-pass filtered is adversely affected by the addition of noise at spatial frequencies in the neighbourhood of the frequencies present in the stereogram. This elevation of threshold is generally termed masking and recent data have been interpreted as evidence for a pair of spatial-frequency-tuned stereo "channels" whose peak spatial frequencies are either at 3 and 5 c/deg or 2.5 and 7 c/deg. This interpretation was re-examined. In particular, we have studied how the characteristics of masking interactions might be affected by taking account of the presence of an initial modulation transfer function (including the optical m.t.f. of the eye) that precedes the stage at which signal and mask interact. Using this approach, we reach the conclusion that the peak of the internal masking function for stereo detection coincides with the signal spatial frequency over the whole range tested (1.7-11.6 c/deg). We conclude that the recent data of Yang and Blake [(1991). Vision Research, 31, 1177-1189] are consistent with a multiple channel model in much the form proposed by Julesz and Miller [(1975). Perception, 4, 125-143]. The analysis presented in this paper has general implications for the interpretation of masking studies in spatial contrast vision.

  • An orientation-tuned component in the contrast masking of stereopsis.

    24 October 2018

    A masking paradigm was used to evaluate the orientation selectivity of the mechanisms mediating human stereopsis. Two experienced and eleven naive observers viewed stereograms, spatially filtered to contain contrast energy with Gaussian passbands in spatial frequency and orientation. Using forced-choice procedures we measured contrast thresholds for stereopsis in the presence of oriented masking patterns. Our results show that the masking of stereopsis consists of two components: one is orientation dependent, the other is non-oriented and has greatest magnitude at lower spatial frequencies. Contrary to an earlier study, these results imply that stereopsis mechanisms may have similar orientation tuning to mechanisms mediating contrast detection.

  • Binocular depth perception and the cerebral cortex.

    24 October 2018

    Our ability to coordinate the use of our left and right eyes and to make use of subtle differences between the images received by each eye allows us to perceive stereoscopic depth, which is important for the visual perception of three-dimensional space. Binocular neurons in the visual cortex combine signals from the left and right eyes. Probing the roles of binocular neurons in different perceptual tasks has advanced our understanding of the stages within the visual cortex that lead to binocular depth perception.

  • A specialization for relative disparity in V2.

    24 October 2018

    Stereoscopic depth perception relies on binocular disparities, or small geometric differences between the retinal images of each eye. The most reliable binocular depth judgments are those that are based on relative disparities between two simultaneously visible features in a scene. Many cortical areas contain neurons that are sensitive to disparity, but it is unclear whether any areas show a specific sensitivity to relative disparity. We recorded from neurons in the early cortical visual area V2 of the awake macaque during presentation of random-dot patterns. The depth of a central region ('center'), and that of an annular surrounding region ('surround'), were manipulated independently in these stimuli. Some cells were fully selective for the resulting relative disparities. Most showed partial selectivity, which nonetheless indicated a sensitivity for the depth relationship between center and surround. Both types of neural response could support psychophysical judgments of relative depth.

  • Two-dimensional spatial structure of receptive fields in monkey striate cortex.

    24 October 2018

    Measurements of the spatial contrast sensitivity function and orientation selectivity of visual neurons in the foveal striate cortex (V1) of primates were interpreted within the context of a model of the two-dimensional spatial structure of their receptive fields. Estimates of the spatial dimensions of the receptive fields along the axis of preferred orientation were derived from the application of the model and were compared with estimates of the smallest spatial subunit in the dimension orthogonal to the preferred orientation. Some measure of agreement was found with corresponding estimates of parameters for psychophysical channels in human foveal vision.

  • Effects of different texture cues on curved surfaces viewed stereoscopically.

    24 October 2018

    Stereoscopic shape judgements can be modified by the addition of texture cues. This paper examines the properties of texture that are responsible for this effect. When a three-dimensional curved surface is projected onto a two-dimensional image, changes in surface orientation result in gradients of texture element size (or area), shape (compression) and density in the image. Manipulating each of these gradients independently we found that 97% of the variance in the results could be accounted for by the compression gradient. When the texture pattern corresponds to a highly anisotropic texture on the object's surface, shape-from-texture becomes ineffective. These results suggest that human shape-from-texture proceeds under the assumption that textures are statistically isotropic, and not that they are homogeneous.

  • Integration of depth modules: stereopsis and texture.

    24 October 2018

    Global shape judgements were employed to examine the combination of stereopsis and shape-from-texture in the determination of three-dimensional shape. Adding textural variations to stereograms increased perceived depth. Thus, texture was not simply vetoed by the strong stereo cue. In experiments where the depth specified by texture was incongruent with that specified by stereo, the data were well described by a weighted linear combination rule. Although only a small weight was assigned to texture, this weight was somewhat greater at a farther viewing distance. This could be a consequence of the decreased reliability of stereopsis at far viewing distances.